1. Field
Aspects of the present disclosure generally relate to high-speed data communications interfaces, and more particularly, to pulse-amplitude-modulation-encoded data-communication links.
2. Background
Manufacturers of mobile devices, such as cellular phones, may deploy various electronic components in one or more integrated circuit (IC) devices and/or on one or more circuit boards. The electronic components may include processing devices, storage devices, communications transceivers, display drivers, and the like. In one example, a processing device may be provided on a printed circuit board (PCB) and may communicate with one or more memories on the same PCB and/or on a different PCB. The processor may communicate with the memory devices using a high-speed communications link that supports unidirectional and bidirectional channels for data and control signals.
In a multi-wire interface, the power consumption associated with a communications link can be significant in relation to a power budget available in a mobile wireless device. The power consumed by the communications link may be determined to some extent by one or more of Bus width, driver types used to transmit data on a transmission line, geometry and structure of the transmission line, encoding format, frequency of switching, logic states, which may be defined by voltage and current levels of an encoding scheme, and so on.
Pulse-amplitude modulation (PAM) is a particular implementation of line coding techniques. PAM signaling can be used in high-speed data transmission schemes to implement data transfers. PAM line coding encodes message symbols (data symbols) into a pulse having an amplitude corresponding to the encoded data symbol. For example, each data symbol may be encoded into a pulse having a unique amplitude. Data symbols may have a one-to-one correspondence in mapping to pulse amplitudes or transmission symbols. PAM signaling on a channel involves the transmission of pulses with various amplitudes to a receiver on a single physical line.
In the PAM receiver, a termination resistor may be coupled between the transmission line and Ground (Gnd) to receive PAM signaling. The termination resistor value is generally selected to match the characteristic impedance of the transmission line and may be implemented as a fixed-value resistor. In some embodiments, the termination resistor could be implemented with an active device like a transistor. As PAM signaling is applied across the termination resistor, the voltage level across the termination resistor is measured to determine the amplitude of a particular PAM pulse. PAM signaling that causes positive and negative values of pulses across the termination resistor is a further aspect of line coding and may be referred to as bipolar encoding. The amplitude of the PAM pulse is used as the index into a lookup table to decode a data symbol that corresponds to the transmission symbol.
In PAM-4 signaling, four discrete pulse amplitudes makeup the encoded transmission symbols. As PAM-4 signaling is applied to the receiver, significant power is lost in the termination resistor. Currently, high speed data links using PAM-4 use a fixed resistor termination scheme with a unidirectional current flow mechanism. Each PAM-4 pulse produces an amount of current in the receiver proportional to the magnitude of the pulse across the termination resistor.